1. Technical Field
The present invention relates to a Class D amplifier, and more particularly, to a Class D amplifier capable of setting restraint power by setting a first reference voltage and restraining power of audio signals according to the level of the first reference voltage.
2. Description of Related Art
Conventional power amplifiers may be classified into Class D power amplifiers and Class AB power amplifiers, among which the Class D power amplifiers are configured to output pulse modulated signals and the Class AB power amplifiers are configured to output linear signals. Owing to the advantage of high efficiency thereof, the Class D power amplifiers have now become a byword for power amplifiers.
Taiwan Patent No. M269646 has disclosed a square-wave modulation scheme for a Class D audio amplifier. Referring to FIG. 1, the circuit diagram of a conventional Class D audio amplifier is shown therein. The Class D audio amplifier is of a differential output configuration formed by a positive output terminal (Out+) and a negative output terminal (Out−). This Class D audio amplifier is advantageous in that by using the differential output to drive a speaker 307, power of the output audio signal can be increased so that the speaker 307 can play the audio sound more efficiently.
Operation of this Class D audio amplifier will now be briefed with reference to an upper half portion of the circuit structure of the Class D audio amplifier (i.e., a single-terminal circuit structure) as follows. From an audio input terminal Ain, an audio signal is inputted into a first operational amplifier 301 and, by means of an integrator consisting of the first operational amplifier 301, a first integrating resistor R16 and a first integrating capacitor Cin1, is transformed into a self-oscillating triangular wave signal. Furthermore, a first square wave signal Vsq1 is inputted into the first operational amplifier 301 via the first resistor Rin1 to synchronize the frequency of the triangular wave signal. Then, a pulse modulated signal is outputted through a second operational amplifier 302 to a first driver 305 which, according to the pulse modulated signal, controls an on/off status of a first transistor set 306 to output an audio signal for driving the speaker 307.
As can be known from the above description of the conventional Class D audio amplifier, the conventional Class D audio amplifier is advantageous in that, a square wave signal is used as an input to control a carrier frequency of the output signal, and design of a square wave circuit is much simpler than that of a triangular wave circuit that is conventionally used. This saves room for circuits in the integrated circuit (IC) design and consequently reduces cost of the circuit layout. However, although the differential output design in which signals are outputted by the positive output terminal (Out+) and the negative output terminal (Out−) can increase power of the output audio signal, there is a risk that the speaker 307 might be damaged due to intolerably high power of the output audio signal because a circuit unit for restraining power amplification is not provided in the circuit structure.
Accordingly, in view of the shortcomings of the conventional Class D audio amplifiers, the inventor of the present invention has made great efforts to make improvement thereon and finally provides a Class D amplifier capable of setting restrained power.